1. Field of the Invention
The invention relates generally to a device for securing a sighting instrument within the bore of a firearm and, more particularly, to an expandable arbor.
2. Description of the Prior Art
Hunting has become increasingly dangerous in recent years, not only for hunters but for people who live and work in suburban areas. With the encroachment of suburbia into former farmlands and woodlands, several states have passed laws limiting the use of weapons and ammunition during certain hunting seasons, particularly deer hunting season. Basically, these laws prohibit the use of high-caliber rifles and require the hunter to use shotguns with slug ammunition only. High-caliber rifles are capable of firing bullets as far as seven miles whereas a shotgun slug will travel a maximum of about 640 yards. Thus, the danger to communities which border on hunting areas is greatly reduced with use of slug ammunition.
This evolution in hunting has created a need for more accurate shotguns. In slug-only jurisdictions, hunters normally mount a scope on the shotgun, and the scope is adjusted for elevation and windage to match the firing characteristics of the particular shotgun on which it is mounted. To accomplish this adjustment, the hunter typically uses a collimator, or "bore sighter", which is mounted on the end of the barrel using an arbor. The arbor is inserted into the bore of the barrel at the muzzle, and it is critical that the arbor be mounted concentrically within the bore to ensure that the scope is properly adjusted.
U.S. Pat. No. 4,057,905 discloses a device for securing a sighting instrument within a shotgun bore. This device has a main shaft 16 which carries a fixed seat 32 on one end with a spring-loaded muzzle stopper 22 on the other. The muzzle stopper has three ball bearing rods 28 extending therefrom, with a ball bearing 30 on the end of each rod 28. The ball bearings are inserted in the muzzle to find the true concentricity of the bore, and the main shaft 16 extends through the muzzle stopper 22 to serve as a means for mounting a collimator thereto. This arrangement has several problems. First, the ball bearings are likely made from steel, which could be harmful to the bore of the barrel, especially if it is rifled. Second, the ball bearing rods 28 must be manufactured and mounted within a very low tolerance to ensure that they are equal in length. A slight deviation in length between one rod and the other can significantly alter the concentricity of the device within the bore. The spring, ball bearing and reciprocating shaft arrangements are complicated and expensive to manufacture. Finally, the spring-loaded main shaft reduces the overall strength of the device and limits the weight of the collimator which is mounted on the main shaft.
Other expandable arbors have been marketed, including Bushnell Model No. 4D10V65 with shotgun adaptor Model No. 9512G30. Similarly, B-Square markets a shotgun bore sighter under Model No. 1BOOSSS. These arbors utilize rubber O-rings as bearing contacts for the shotgun bore. The O-rings are easily damaged when the arbor is compressed into a rifled shotgun barrel or a removable choke tube barrel. Once the O-ring is cut or scratched in any way, it will not project the true concentricity of the bore, therefore causing improper adjustment of the scope.
To date, there are no expandable arbors for use with black powder rifles and shotguns having bore sizes ranging from 0.490 to 0.800 inch.
It is therefore an object of the present invention to provide an arbor which will accommodate a wide range of bore sizes, including those for black powder rifle barrels, shotgun barrels and choke tubes. It is a further object to provide an inexpensive expandable arbor which is not easily damaged in use and which offers maximum accuracy in finding the true bore axis.